The objectives of the project on bone metabolism are to investigate the factors influencing mineral storage and loss in demineralizing bone diseases, to investigate the response of the parathyroid gland to rapid changes in blood calcium, and to study the influence of added calcium and phosphate on resorption of bone mineral. The project involves metabolic balance studies. It also involves estimation of pool sizes, turnover rates, bone deposition rates, and absorption rates of calcium in patients by the oral and intravenous administration of tracer doses of 47Ca, and estimation of bone density by photon absorptiometry. During the past year, clinical studies of osteoporosis have been continued, using photon absorption densitometry, with observations of the bone mineral content in various populations and of the possible effect thereon of dietary manipulation.